The ornamental design for a containerbase, as shown and described.
FIG. 1 is a top isometric view of a container base, showing my new design;
FIG. 2 is a right side elevational view thereof, the left side elevational view being a mirror image thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a front elevational view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines in the figures illustrate the portions of the design that form no part of the claimed design. The broken lines lid in the figures illustrate environmental subject matter that form no part of the claimed design. The heavy broken lines on the rear tab in FIGS. 4 and 6 are included to show unclaimed perforations on the article.